1. Field of the Invention
The present invention relates to equipment and a method for fabricating a semiconductor device, and more particularly, to equipment and a method for measuring a transmittance of a photomask used in a photolithography process.
2. Description of the Related Art
Photomasks are used in microchip fabrication to form tiny circuit patterns on semiconductor substrates. Such photomasks typically include a light transmissive substrate such as quartz having deposited thereon a pattern of chrome defining a light blocking pattern.
In photolithography manufacturing, wafer circuit patterns are manufactured by projecting a pattern of light onto the photosensitive coating atop a wafer. The light pattern is formed by interposing the photomask between the light source and the wafer. Focused light from the source passes through the transmissive portions of the photomask but are blocked by the chrome pattern on the photomask.
Continuing manufacturing advances have decreased the size of these circuits and the critical dimensions (CD) of the circuit and photolithographic patterns. Several techniques have been developed to address problems with manufacturing semiconductor devices with small critical dimensions.
In a first technique, femto-second pulses from a laser are used to create correcting elements within the photomask that improve the uniformity of light transmission across the entire photomask. Improved uniformity is key to ensuring precise formation of patterns on the photomask and prevention of errors during the manufacturing process.
The correcting elements have varying characteristic depending upon the need. For example, the correcting elements may have a diameter of about 1 μm and a depth of about 30 μm. Furthermore, the correcting elements may be formed in a repeated structure and pitches of the correcting element may be adjusted to adjust the transmittance of the light transmissive substrate. Regions of the correcting elements with a small pitch have denser correcting elements, thereby lowering the transmittance compared to regions using correcting elements with a large pitch. A relationship between the pitches of the correcting elements and the transmittance of the light transmissive substrate can be observed using transmittance measuring equipment in exposure equipment for fabricating a photomask.
The second technique for reducing errors in small critical dimension photomasks is the use of off-axis illumination (OAI). Whereas conventional illumination through the photomask has been perpendicular to its surface, illumination through the photomask using off-axis light has been found to result in a higher resolution and a deeper depth-of-focus (DOF) margin.
Conventional techniques for OAI include either tilting the laser with respect to the photomask surface, or tilting the photomask itself. Each of these techniques, however, has a drawback in that the stage used to mount either the tilting laser or tilting photomask must be controlled with extremely tight tolerances. As a result, it has been extremely difficult to achieve accurate control of off-axis illumination through the photomask to measure the uniformity of its transmittance.
Accordingly, the need remains for methods and system which achieve more accurate control of OAI through a photomask.